1. Field of the Invention
The present invention relates to a stabilizer and a stabilizing method for an aqueous aliphatic aldehyde solution. More particularly, the invention relates to a stabilizer and a stabilizing method for preventing turbidity or precipitation in an aqueous aliphatic aldehyde solution in storage.
2.Description of Related Art
Aqueous aliphatic aldehyde solutions tend to generate turbidity or precipitates while they are stored. This nature of the aqueous aliphatic aldehyde solutions makes it very difficult to handle the aqueous aliphatic aldehyde solutions. Formaldehyde, for example, is mostly hydrated in an aqueous solution and exists as an equilibrium mixture of methylene glycol and lower polyoxymethylene glycol. This equilibrium shifts to a higher polymerization as the concentration of formaldehyde increases. A highly polymerized component precipitates as paraformaldehyde which is insoluble in water. This formation of paraformaldehyde results in the turbidity or precipitation of the aqueous formaldehyde solution and, in extreme cases, results in solidification of the whole solution. The aqueous formaldehyde solution is thus poor in stability and very difficult to handle. The turbidity or precipitates cause apparatus and piping of a chemical plant to be blocked. Furthermore, in recent years, a highly concentrated aqueous formaldehyde solution of 50 wt % or more has become industrially used (conventional solutions generally have a concentration of about 37 wt %), and therefore a solution to the above-mentioned problem has eagerly been sought.
In order to prevent the generation of paraformaldehyde during the storage of an aqueous formaldehyde solution, a number of ways have been disclosed in Japanese Patent Publications (Kokoku) Nos. Sho 57(1982)-61009, Sho 62(1987)-41593, Hei 8(1996)-2824 and Hei 8 (1996)-2825 and Japanese Unexamined Patent Publications (Kokai) Nos. Hei 4(1992)-330030 and Hei 4(1992)-330031, in which methanol content in an aqueous formaldehyde solution is increased, storage temperature is raised or various additives are added to the solution.
For another example, glutaraldehyde is a known compound as a purifying agent, disinfectant or sterilant. In order to ensure such effect, an aqueous glutaraldehyde solution must be kept at a weakly alkaline pH of about 7.5 to 8.5. This aqueous glutaraldehyde solution adjusted to the weakly alkaline side also generates turbidity or precipitates when allowed to stand at room temperature or above. Particularly in a medical field, the above-described precipitates enter into gaps of, or adhere to, a surface of medical instruments such as endoscopes and disinfection apparatus for endoscopes, for example. In order to prevent the turbidity and precipitation in the aqueous glutaraldehyde solution, a variety of stabilizers have been disclosed in Japanese Patent Publication (Kokoku) No. Hei 5(1993)-29337 and Japanese Unexamined Patent Publications (Kokai) Nos. Sho 57-195463 and Hei 7(1995)-69956.
The methanol content in the aqueous formaldehyde solution is wished to be decreased in recent years for economic needs, for example, for reducing costs, and for users' needs, for example, for decrease of reaction time in synthesis of resins and for reduction of the burden of disposing of industrial waste water. However, in order to stabilize an aqueous formaldehyde solution containing a reduced amount of methanol., the use amount of a stabilizing agent must be increased and the storage temperature must be raised.
As for the aqueous glutaraldehyde solution, there is a demand for a stabilizer which permits long-term storage in weak alkali.